One example of gas sensor based on measuring the spectrum absorptions of emitted light by a gas is described in US 2008/0283753, wherein the pass band of a first filter is arranged within the pass band of a second filter and the evaluating device forms the difference of the signals and normalises it to the signal.
Using such a sensor in relatively harsh environments such as exhaust systems in ships, vehicles etc. will however expose the delicate parts environment within the exhaust stack that may comprise a wide range of particles and gasses that could damage them, or just reduce their lifetime. One option would be to protect the parts with sight glasses such that they becomes isolated from the harsh environment, but the transparency of these may then be reduced over time by settlement of particles etc.
Another example of a gas sensor is CA 984173 disclosing a photometer disclosing a probe with a mirror at the end such that light is passed down the probe to the mirror and is reflected back to the instruments of the device. The probe has many holes to allow free passage of the gasses and air lines to purge the probe and for calibrating. The purge gasses enter from an externally positioned section covering holes for the purge gas to be feed into the measuring region. An disadvantage with the this construction is e.g. the free passage of gasses to the measuring region making it hard to empty the measuring region from gasses during calibration as it would require a significant pressure to overcome the forces of pressure from the freely flowing gasses.
Another example is DE 10 2012 215 805 showing a split system where two parts of the sensor is positioned at opposite sides of the gas containing region making it even more impossible to empty the measuring region (being the gas containing region) from gasses during a possible calibration.